Optical sensor, optical current sensor and optical voltage sensor

ABSTRACT

[Object] A simple constitution together with an easy calibration of output by realizing a fast light intensity detection method is realized without using the carrier signal. 
     [Solution] An optical sensor, including: a sensor to which light from a light source is lead, and by which light intensity of the light is modulated based on a physical value; light receiving elements  61  and  62  receiving two elements of divided light PA and PB having polarized waves which are orthogonally crossing each other; a variable optical attenuator operating light which is received by the light receiving elements  61  and  62 ; and a variable amplifier operating output signals from the light receiving elements  61  and  62 , wherein both a zero point of a sensor output and sensitivity are calibrated based on a light attenuation factor or an amplification factor which is adjusted when a physical value is detected by calculating a ratio between a sum and a difference of outputs of the light receiving elements  61  and  62.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical sensors, especially an opticalcurrent sensor and an optical voltage sensor which can measure a widerange of current and voltage from a DC to a high frequency AC.

2. Description of the Related Art

In recent years, developments and practical applications of an opticalfiber AC sensor by using the Faraday effect have been proposed (forexample, see non-patent document 1: Kurosawa “Development andApplication of Optical Fiber Current Sensor”, Journal of the Instituteof Electrostatics Japan, vol. 28 No. 5 (p. 251-257), 2004). Along withthe development of the optical fiber AC sensor, it has been expected torealize an optical fiber DC sensor that can be applied to both a powerelectronics machine and a DC transmission facility/DC substationequipment using the power electronics machine.

With respect to such a DC sensor, it is necessary to detect not only aDC (component having zero frequency), but also both a DC generated fromsuperimposed components of high frequency and a current having a shortrise time (1 msec or less, in some cases, 1 μsec). However, with respectto these necessities, there are problems such as an application oftechnologies developed for AC, a method of setting a zero point (amethod of setting an output 0 when a measured current is 0), a method ofadjusting the sensitivity (adjusting a reset value of the sensitivity ofoutput) and moreover the stabilization of set values. Regarding anintensity modulation type AC sensor, as described in non-patent document1, these problems have been solved by using a method of calculating adegree of modulation of a received signal.

However, it is not possible to apply the method of calculating thedegree of modulation to a DC. Therefore, for example, a method of usinga Sagnac interferometer which is used in a fiber optic gyro (forexample, see non-patent document 2: M. Takahashi, et al. “SagnacInterferometer-type fiber-optic current sensor using single-mode fiberdown leads” Technical Digest of 16th International conference on opticalfiber sensor) and a method of using an optical heterodyne (for example,see non-patent document 3: Kurosawa “Study of FundamentalCharacteristics of Photocurrent Transformer using Optical HeterodyneMethod”, volume B117, No. 3 (p. 354-363), 1997, The Institute ofElectrical Engineers of Japan, Transactions on Power and Energy) arecurrently developed. FIG. 8 is a drawing which shows a method describedin non-patent document 2, and an outline of the method is explainedbelow.

Light emitted from a light source is converted to be linear polarizedlight after passing through a coupler 1, a depolarizer and a lightpolarizer. The linear polarized light is divided into a pair of beams bya coupler 2, and the beams pass into a loop fiber interferometer asincident light. The beams rotate in opposite directions to each otherinside a loop. The pair of the beams is propagated through a sensorfiber after being converted to circularly polarized waves by aquarter-wave plate. In this step, a magnetic field induce by a measuredcurrent is applied to the sensor fiber, and a difference of propagationvelocities between the pair of the beams is caused because of theFaraday effect. The pair of the beams reaches light receiving elementsafter being optically multiplexed by the coupler 2, and a phasedifference, in other words, an intensity difference of received lightaccording to a current is caused. Values of the current are calculatedbased on this intensity difference of received light.

A system described above has a constitution in which, in order tomaintain the sensitivity and in order to maintain the stability ofoutput even if the intensity of received light fluctuates, a carriersignal is generated by modulating light by using a piezoelectricvibrator (PZT), and the carrier is further modulated because of theFaraday effect. As such, it is possible to obtain a system output bydemodulating the carrier. In other words, in FIG. 8, a reference signalgenerator, a vibrator activating portion and a piezoelectric vibratorare provided in order to generate the modulated signal.

Moreover, in order to improve the efficiency of modulation, an appendedfiber (for example, approximately 100 m) is provided. Furthermore, inorder to maintain the depth of modulation so as to be a certain level,after extracting second-harmonic waves and fourth-harmonic wavesincluded in the received signal, a ratio between second-harmonic wavesand fourth-harmonic waves is calculated and a signal which isproportional to the ratio is output to a modulation circuit in order toconduct a feedback operation. Regarding a signal operation portion, itis necessary to provide a synchronous detector and the like, which arerather complex components.

In a case of using a Sagnac interferometer, it is necessary to conductmodulation because if the modulation is not conducted, the sensitivityof the system is theoretically zero when a current is small. Moreover,in this case, in order to maintain a measurement accuracy of the system,other than modulation and demodulation, it is necessary to selectoptical components such as a ¼ plate, a polarizer and a depolarizerwhich have high accuracy.

As described above, a Sagnac interferometer has the following problems.

i) A delicate and complex optical system is necessary.ii) A complex signal operation circuit is necessary.iii) It is difficult to improve a response speed because complexoperations such as generating the carrier signal and a modulation anddemodulation are conducted. In order to improve a response speed, it isnecessary to increase the frequency of the carrier. However, in thiscase, the modulation power increases and pressure on the signaloperation circuit increases too.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a simpleconstitution together with an easy adjustment of zero point and an easycalibration of output by realizing a light intensity detection method inwhich information is indicated by using the intensity of a high speed(high response speed, short rise time) received signal without using thecarrier signal.

In order to solve these problems, the invention of Claim 1 ischaracterized by including: a sensor constituted from an optical portionto which light from a light source is lead, and by which the lightintensity of the light is modulated based on a physical value; a firstlight receiving element and a second light receiving element receivingtwo elements of divided light having polarized waves which areorthogonally crossing each other; a variable optical attenuatoroperating light which is received by the first and the second lightreceiving elements; and a variable amplifier operating output signalsfrom the first and the second light receiving elements, wherein both azero point and sensitivity of a sensor output can be calibrated based ona light attenuation factor or an amplification factor which is adjustedwhen a physical value is detected by calculating a ratio between a sumand a difference of outputs of the first and the second light receivingelements.

The invention of Claim 2 is characterized by including: a sensorconstituted from an optical portion to which light from a light sourceis lead, and by which light intensity of the light is modulated based ona physical value; a first light receiving element and a second lightreceiving element receiving two elements of divided light havingpolarized waves which are orthogonally crossing each other; a thirdlight receiving element directly receiving light from the light source;a variable optical attenuator operating both light which is received byone of the first and the second light receiving elements and lightreceived by the third light receiving element; and a variable amplifieroperating both an output signal from one of the first and the secondlight receiving elements and an output signal from the third lightreceiving element, wherein both a zero point and sensitivity of a sensoroutput are calibrated based on a light attenuation factor or anamplification factor which is adjusted when a physical value is detectedby calculating a ratio between the output of the third light receivingelement and a difference between one of the outputs from the first andsecond light receiving elements and the third light receiving element.

It is possible to obtain an optical current sensor by applying theprinciple of the Faraday effect to the above-described inventions ofClaims 1 and 2 (invention of Claim 3), and it is possible to obtain anoptical voltage sensor by applying the principle of Pockels effect tothe above-described inventions of Claims 1 and 2 (invention of Claim 4).

In accordance with the present invention, a light intensity detectionmethod in which information is indicated by using the intensity of ahigh speed (high response speed, short rise time) received signalwithout using the carrier signal is realized. Therefore, a significantlysimple constitution together with an easy adjustment of zero point andan easy calibration of output by realizing is achieved. As a result, anadvantage is obtained in which it is possible to measure a wide range ofcurrent and voltage from a DC to a high frequency AC.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing which explains a principle of the present invention.

FIG. 2 is a drawing which explains a relationship among P_(A), P_(B) andθ_(F) of FIG. 1.

FIG. 3 is a block diagram showing one embodiment of the presentinvention.

FIG. 4 is a block diagram showing another embodiment of the presentinvention.

FIG. 5 is a block diagram which can be applied to both FIGS. 3 and 4.

FIG. 6 is a drawing showing a system constitution of a two-signalmethod.

FIG. 7 is a drawing showing a system constitution of a one-signalmethod.

FIG. 8 is a block diagram showing a Sagnac interferometer as aconventional example.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a drawing which shows a principle constitution of the presentinvention.

In FIG. 1, 1 is a light source, 2 is a beam splitter, 3 is a lightpolarizer, 4 is a Faraday cell, 5 is an analyser, and 61-63 are lightreceiving elements such as photodiodes (PD).

Now, a separation ratio of the beam splitter 2 is R, a transmissionefficiency regarding a quantity of light is η, a length of the Faradaycell 4 is L, a Faraday rotation angle is θ, a Verdet constant is V andan intensity of a magnetic field generated by a current I is H.Followings are defined.

|2θ_(F)|<<π/2, θ_(F)=VHL=VI  (1)

Quantity of light P_(A), P_(B) and P_(R) which reach the light receivingelements 61-63 are shown by using the following equations.

P _(A)=η₁*η₁*η_(A)*(1−R)(1/2)P ₀(1+2θ_(F))  (2)

P _(B)=η₁*η₂*η_(B)*(1−R)(1/2)P ₀(1-2θ_(F))  (3)

P _(R)=η₁*η_(R) *R*P ₀  (4)

In an ideal case, the following equations are applied.

R=0  (5)

η₁=η₂=η_(R)=η_(A)=η_(B)=1  (6)

Therefore, the following equations are obtained by applying equations(5) and (6) to equations (2)-(4).

P _(A)=(1/2)P ₀(1+2θ_(F))  (7)

P _(B)=(1/2)P ₀(1−2θ_(F))  (8)

P_(R)=0  (9)

(1) Two-Signals Method

This is a method which uses both signals P_(A) and P_(B). Here, equation(7)+equation (8) is obtained as follows.

P _(A) +P _(B) =P ₀  (10)

P _(A) −P _(B) =P ₀*2θ_(F)  (11)

Therefore, a degree of modulation M₂ is shown as follows.

M ₂=(P _(A) −P _(B))/(P _(A) +P _(B))=2θ_(F)  (12)

FIG. 2 shows a relationship among P_(A), P_(B) and θ_(F).

In other words, in an ideal situation, it is possible to maintainaccuracy by outputting the degree of modulation M₂ as a sensor output.However, in a practical case, η₁, η₂, η_(R),η_(A), η_(B)≠1. Therefore,the degree of modulation M₂ is obtained as follows by applying R=0 basedon the above-described equations (2) and (3).

M ₂=(P _(A) −P _(B))/(P _(A) +P_(B))={η_(A)(1+2θ_(F))−η_(B)(1−2θ_(F))}/{η_(A)(1+2θ_(F))+η_(B)(1−2θ_(F))}  (13)

Therefore, it is clearly understood that M₂≠2θ_(F) unless η_(A)=η_(B).

Therefore, it is not possible to maintain the accuracy of the sensoroutput if M₂ is directly calculated as shown in equation (12). Thefollowing equations are obtained by multiplying coefficients G_(A) andG_(B) and the amount of light P_(A) and P_(B) radiated on the lightreceiving elements 61 and 62.

P_(A)′=G_(A)P_(A)  (14)

P_(B)′=G_(B)P_(B)  (15)

The degree of modulation M₂ is obtained as follows by applying P_(A) andP_(B) of the above-described equation (13).

M ₂ ={G _(A)η_(A)(1+2θ_(F))−G _(B)η_(B)(1−2θ_(F))}/{G_(A)η_(A)(1+2θ_(F))+G _(B)η_(B)(1−2θ_(F))}  (16)

Now, an equation G_(A)η_(A)=G_(B)η_(B) . . . (17) is applied to (16),and the following equation is obtained.

M₂=2θ_(F)  (18)

It is possible to adjust G_(A) and G_(B) so as to have M₂=0 if θ_(F)=0

(2) One-Signal Method

This method is a method in which one signal is used. Here, for example,P_(A) is used and the following equation (7) shows P_(A).

P _(A)=(1/2)P ₀(1+2θ_(F))  (7)

P₀/2 is subtracted from the above-described equation (7) in order toobtain the degree of modulation M₁.

M ₁ =P _(A) −P ₀/2=θ_(F)  (19)

According to the above-described equation, in an ideal status, it ispossible to maintain the accuracy by using the degree of modulation M₁as the sensor output. However, in practical cases, η₁,η₂,η_(A)≠1.Therefore, when R=0, the degree of modulation M₁ is obtained as followsbased on the above-described equation (2).

M ₁=η₁η₂η_(A)*(1/2)P ₀(1+2θ_(F))−(1/2)P ₀=(1/2)P₀{η₁η₂η_(A)(1+2θ_(F))−1}  (20)

In the above-described the equation, it is clear that M₁≠θ_(F) unlessη₁η₂η_(A)=1. Therefore, it is not possible to maintain accuracy of thesensor output even if M₁ shown by the equation (19) is calculated.Therefore, the amount of light P_(A) radiating on the light receivingelement 61 is multiplied by a coefficient G_(A), and moreover, areference signal P_(R) which has a proportional relationship with P₀ isapplied in order to take account of the difficulty of stabilizing alight source in regular cases. In this case, the amount of light P_(R)radiating on the light receiving element 63 is multiplied by acoefficient G_(R) as well. That is, as follows.

P_(A)′=G_(A)P_(A)  (21)

P_(R)′=G_(R)P_(R)  (22)

The degree of modulation M1 is obtained by applying the above-describedequations (2) and (4) to the equations (21) and (22).

$\quad\begin{matrix}{M_{1} = {{{\begin{Bmatrix}{G_{A}\eta_{1}\eta_{2}{\eta_{A}\left( {1 - R} \right)}\left( {1/2} \right)} \\{{P_{0}\left( {1 + {2\; \theta_{F}}} \right)} - {G_{R}\eta_{1}\eta_{R}{RP}_{0}}}\end{Bmatrix}/G_{R}}\eta_{1}\eta_{R}{RP}_{0}}\mspace{34mu} = {{\left\{ {{G_{A}\eta_{2}{\eta_{A}\left( {1 - R} \right)}\left( {1/2} \right)\left( {1 + {2\; \theta_{F}}} \right)} - {G_{R}\eta_{R}R}} \right\}/G_{R}}\eta_{R}R}}} & (23)\end{matrix}$

Now, the following equation is applied to the equation (23).

(1/2)G _(A)η₂η_(A)(1−R)=G _(R)η_(R) R=K  (24)

M ₁={2Kθ _(F)+(K−K)}/K=2θ_(F)  (25)

It is possible to adjust G_(A) and G_(R) so as to have M₁=0 if θ_(F)=0

It is necessary to take account of conditions as follows in order toconstitute an apparatus with characteristics including: an intensitydetection method having a simple constitution; functions of setting zeropoint and adjusting sensitivity; stabilization of set value; quickresponse; and the like.

i) A value of the received light signal is determined based on both anintensity of the light source and a transmission efficiency of anoptical path from the light source to the light receiving elements.These parameters are not determined when the apparatus is assembled butare respectively different with regard to the optical systems.ii) Because it is necessary to apply a single mode fiber with a smallcore diameter between the light source and an optical fiber currentelement, the quantity of light radiated on sensor elements is easilyaffected from small differences of the optical systems. On the otherhand, it is possible to apply a multi-mode fiber with a large diameterfor transmitting the light to the light receiving element after passingthrough the analyser which is arranged very close to the sensor element.Therefore, if difficulties are compared between stabilization of thequantity of light radiated on the sensor element and stabilization oftransmission efficiency of the light to the light receiving elementafter passing through the analyser, stabilization of the quantity oflight radiated on the sensor element is more difficult than the laterone.

Based on the above-described studies, it is understood that thefollowing is effective for constituting a DC detection apparatus usingintensity modulation.

a) The optical system and the signal operation system are arranged andadjusted in a manner in which values of the signals are adjusted byusing optical or electrical methods in order to calibrate the output soas to be zero when the measured current is zero.b) The optical system and the signal operation system are arranged andadjusted in order to prevent fluctuation of the output with regard tothe quantity of light radiated on the sensor elements.

Therefore, in the present invention, the following solutions areapplied. FIG. 3 is a drawing which shows a constitution of an embodimentof the present invention. FIG. 3 shows an example of a basicconstitution of signal operation which can adjust the coefficientsG_(A), G_(B) and G_(R) so as to satisfy the equation (17) of thetwo-signal method.

For example, the quantity of light P_(A) and P_(B) obtained by dividingthe sensor output light into two elements having polarized waves whichare orthogonally crossing each other, is respectively lead to the lightreceiving elements (PD) 61 and 62 via a variable optical attenuators(ATT) 71 and 72. Variable amplifiers (G) 91 and 82 are inserted withregard to the output signals from the light receiving elements (PD). Itshould be noted that 91 is a subtractor, 92 is an adder, 10 is a divider(DIV).

In the adjustment method, (1) in a state in which the measuredcurrent=0, (2) attenuation factors α_(A) and α_(B) of ATT 71 and 72 oramplification factors g_(A) and g_(B) of G81 and 82 are adjusted so asto set the output S=0. In accordance with such a method, the equation(17) is satisfied. In other words, G_(A)η_(A)=G_(B)η_(B) . . . (17) issatisfied. The following equations should be noted.

G_(A)=α_(A)g_(A)  (26)

G_(B)=α_(B)g_(B)  (27)

It should be noted that it is not necessary to adjust all ofα_(A),α_(B),g_(A) and g_(B) in order to satisfy (17), (26) and (27). Itis possible to adjust at least one of 4 parameters.

FIG. 4 is a drawing which shows a constitution of another embodiment ofthe present invention. FIG. 4 shows an example of a basic constitutionof a signal operation which can adjust the coefficients G_(A), G_(B) andG_(R) so as to satisfy the equation (24) of the one-signal method.

For example, both a reference beam P_(R) and one of the quantity oflight P_(A) and P_(B) obtained by dividing the sensor output light intotwo elements having polarized waves which are orthogonally crossing eachother, are lead to the light receiving elements (PD) 61 and 63 viavariable optical attenuators (ATT) 71 and 73. Variable amplifiers (G) 81and 83 are inserted with regard to the output signals from the lightreceiving elements (PD). It should be noted that 11 is a low-pass filter(LPF) for canceling effects which are caused if AC elements such as aripple are included in the reference signal.

The adjustment method is the same as FIG. 3. In the adjustment method,(1) in a state in which the measured current=0, (2) attenuation factorsα_(A) and α_(B) of ATT 71 and 73 or amplification factors g_(A) andg_(B) of G81 and 83 are adjusted so as to set the output S=0. Inaccordance with such a method, the equation (24) is satisfied. In otherwords, the following equation is satisfied.

(1/2)G _(A)η₂η_(A)(1−R)=G _(R)η_(R) R=K  (24)

Here, the following should be noted.

G_(A)=α_(A)g_(A)  (28)

G_(R)=α_(R)g_(R)  (29)

It should be noted that it is not necessary to adjust all ofα_(A),α_(R),g_(A) and g_(R) in order to satisfy (24), (28) and (29). Itis possible to adjust at least one of 4 parameters.

FIG. 5 shows an example of a circuit which can be applied to both FIG. 3and FIG. 4.

In this case, one of the signals P_(A) and P_(B) (here, P_(A)) is leadto the upper side of the circuit and P_(B) or P_(R) is selectively leadto the lower side of the circuit. Two switches SW are respectivelyprovided at positions shown in the drawing. Therefore, if the signalP_(B) is lead to the lower side of the circuit and the switch SW isturned on, it is possible to obtain a circuit shown in FIG. 3. Moreover,if the signal P_(R) is lead to the lower side of the circuit and theswitch SW is turned on, and it is possible to obtain a circuit shown inFIG. 4.

FIGS. 6 and 7 show an overall constitution including the above-describedsensor optical system and the above-described signal operation system.

FIG. 6 shows an example of the two-signal method, and FIG. 7 shows anexample of the one-signal method. In FIG. 6, (a) is a transmission type,and (b) is a reflection type. In FIG. 7, (a) is a transmission type, (b)is a reflection type (1), and (c) is a reflection type (2). Theirfunctions and effects are the same as described above. Therefore, anexplanation is omitted. It should be noted that, in FIG. 6 (b) and FIG.7 (b) and (c), a reference numeral 12 is a mirror, 13 is apolarizer/analyser (including functions of both a polarizer and ananalyser).

It should be noted that in the above description, the current sensor ismainly explained. However, the present invention can provide an opticalvoltage sensor by applying Pockels effect in place of the Faradayeffect. Therefore, it is possible to provide an optical sensor whichdetects physical values including current and voltage by applying thepresent invention.

1. An optical sensor, comprising: a sensor constituted from an opticalportion to which light from a light source is lead, and by which lightintensity of the light is modulated based on a physical value; a firstlight receiving element and a second light receiving element receivingtwo elements of divided light having polarized waves which areorthogonally crossing each other; a variable optical attenuatoroperating light which is received by the first and the second lightreceiving elements; and a variable amplifier operating output signalsfrom the first and the second light receiving elements, wherein both azero point and sensitivity of a sensor output are calibrated based on alight attenuation factor or an amplification factor which is adjustedwhen a physical value is detected by calculating a ratio between a sumand a difference of outputs of the first and the second light receivingelements.
 2. An optical sensor, comprising: a sensor constituted from anoptical portion to which light from a light source is lead, and by whichlight intensity of the light is modulated based on a physical value; afirst light receiving element and a second light receiving elementreceiving two elements of divided light having polarized waves which areorthogonally crossing each other; a third light receiving elementdirectly receiving light from the light source; a variable opticalattenuator operating both light which is received by one of the firstand the second light receiving elements and light received by the thirdlight receiving element; and a variable amplifier operating both anoutput signal from one of the first and the second light receivingelements and an output signal from the third light receiving element,wherein both a zero point and sensitivity of a sensor output arecalibrated based on a light attenuation factor or an amplificationfactor which is adjusted when a physical value is detected bycalculating a ratio between the output of the third light receivingelement and a difference between one of the outputs from the first andsecond light receiving elements and the third light receiving element.3. An optical sensor according to claim 1, wherein the optical sensoruses a Faraday effect.
 4. An optical sensor according to claim 1,wherein the optical sensor uses Pockels effect.
 5. An optical sensoraccording to claim 2, wherein the optical sensor uses a Faraday effect.6. An optical sensor according to claim 2, wherein the optical sensoruses Pockels effect.